1. Technical Field
The invention relates to a plain bearing, comprising a backing member and at least one metallic overlay, which is applied by means of electron beam vapour deposition. The invention also relates to a method of producing a plain bearing having such an overlay according to the precharacterising clause of claim 4.
2. Related Prior Art
Examples of plain bearings are radial bearings, axial bearings and axial/radial bearings, the functional area being present in flat or curved form and coating being effected in the finished state or during an intermediate stage, such as at the semifinished product stage for instance (strips or strip portions of flat form).
In general, plain bearings used for such purposes comprise multilayer composite systems of the following construction: steel support member serving as backing material, bearing metal layer of a Cu, Al or white metal alloy and a so-called sliding or third layer or overlay, which may be applied either by an electroplating process (E. Romer: Three-component bearings of GLYCO 40; GLYCO Engineering Report 8/67) or by a cathodic sputtering process as described in EP 0 256 226 B1. Layers applied by electroplating, which are generally based on Pb or Sn, exhibit the disadvantages of frequently inadequate corrosion resistance and low wear resistance. Furthermore, the electroplating process is in itself dubious from the environmental point of the view.
Where overlays are applied by the sputtering method, a considerable cost factor is introduced with respect to the complete plain bearing, owing to the low deposition rates achievable therewith and the high technical complexity of the equipment needed.
Moreover, it is known from DE 43 90 686 T1 that these overlays with special surface structures (pyramid-shaped crystal grains on the surface of the Pb layer deposited by electroplating) exhibit excellent blocking and fatigue strength. This situation may be put down to good oil retention and to the dispersion and reduction of a concentratedly applied load by the pyramid-shaped crystal grains of the surface. Disadvantages of this method are that once again an electroplating process is used, with all the attendant shortcomings, and the process for producing this special surface structure is very expensive, since it is a multistage process with additional heat treatment. Furthermore, the surface layer consists of a lead alloy of questionable toxicity.
DE 196 08 028 A1 likewise describes a special sliding surface structure, which has a positive effect on sliding properties. The corrosion sensitivity of the surface is countered on the one hand by hexagonal pyramid-shaped metal crystals in the surface or by the inclusion of oxygen, phosphorus etc in the sides of the pyramids (hardening effect). This method is used exclusively for iron-based alloys, the specially structured sliding surface consisting of Fe crystals. A typical application is the coating of piston pins. Their tribological properties render such alloys unsuitable for use for plain bearings.
It is also known from DE 195 14 835 A1 and 195 14 836 A1 to deposit overlays on concavely curved plain bearings by means of electron beam vapour deposition. Using this method it is possible, by adjusting certain method parameters, to produce specific layer thickness profiles over the circumference of the plain bearing. No reference is made in these documents to a special surface topography which may be produced using this method. However, for a plurality of applications the tribological properties achievable therewith are inadequate.
A method is known from DE 36 06 529 A1 for producing multilayer materials or multilayer workpieces by the vapour deposition of at least one metallic material onto a metallic substrate, an electron beam vapour deposition process likewise being used to apply the overlay. The method is carried out in a residual gas atmosphere under pressures ranging from 10.sup.-2 -10.sup.-3, wherein the material is dispersion-hardened or dispersion-strengthened simultaneously with the vapour deposition. Coating rates are set at approximately 0.3 .mu.m/s. During vapour deposition, the substrate is kept at a temperature between 200.degree. C. and 800 .degree. C. The temperature of the substrate is 200.degree. C. to 300.degree. C. for vapour deposition of aluminium alloys and in the range of from 500.degree. C. to 700.degree. C. for vapour deposition of copper-lead alloys. No mention is made of the topography of the overlays produced using this method. The load-carrying capacity of the layers produced according to this method is markedly better than that of layers produced by powder-metallurgical methods. In many instances of application, plain bearings produced according to this method do not exhibit satisfactory wear resistance. A priority of this application is to produce a defined hard phase content in the overlay by dispersion strengthening, e.g. by producing oxides during vapour deposition. Optimisation of the surface shape is not mentioned.